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Differential Regulation of Steroid Hormone Biosynthesis in R2C and MA-10 Leydig Tumor Cells: Role of SR-B1-Mediated Selective Cholesteryl Ester Transport¹

^a Department of Cell Biology and Biochemistry, Texas Tech University Health Sciences Center, Lubbock, Texas 79430 ^b Geriatric Research, Education and Clinical Center, VA Palo Alto Health Care System, Palo Alto, California 94304 ^c Department of Pediatrics and the Metabolic Research Unit, University of California, San Francisco, San Francisco, California 94143-0978

The rat R2C Leydig tumor cell line is constitutively steroidogenic in nature, while the mouse MA-10 Leydig tumor cell line synthesizes large amounts of steroids only in response to hormonal stimulation. Earlier studies showed abundant cAMP-independent steroid production and constitutive expression of steroidogenic acute regulatory (StAR) protein in R2C cells. The objective of the current study was to identify possible genetic alterations in the R2C cell line responsible for rendering it a constitutively steroidogenic cell line, especially those that might have altered its cholesterol homeostatic mechanisms. Measurement of the levels of cholesterol esters and free cholesterol, precursors for steroidogenesis, indicated that R2C mitochondria were fourfold enriched in free cholesterol content compared with MA-10 mitochondria. In addition to the previously demonstrated increased expression of StAR protein, we show that R2C cells possess marginally enhanced protein kinase A activity, exhibit higher capacity to take up extracellular cholesterol esters, and express much higher levels of scavenger receptor-type B class 1 (SR-B1) and hormone sensitive lipase (HSL). These observations suggest that the high level of steroid biosynthesis in R2C cells is a result of the constitutive expression of the components involved in the uptake of cholesterol esters (SR-B1), their conversion to free cholesterol (HSL), and its mobilization to the inner mitochondrial membrane (StAR).

¹ This work was supported by NIH grant HD-17481 and funds from the Robert A. Welch Foundation (to D.M.S.), NIH grant DK-56339 and funds from the Office of Research and Development, Medical Research Service, Department of Veterans Affairs (to S.A.), and NIH grant DK37922 (to W.L.M.).

² Correspondence. FAX: 806 743 2990; doug.stocco{at}ttmc.ttuhsc.edu

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